This invention relates to processes for making polycrystalline abrasive compacts and, more particularly, it relates to a direct conversion process for making a cubic boron nitride (CBN) compact from hexagonal boron nitride (HBN) and the resulting product.
Three crystalline forms of boron nitride are known: (1) soft graphitic (hexagonal) form (HBN) similar in structure to graphite carbon, (2) a hard wurtzitic (hexagonal) form (WBN) similar to hexagonal diamond, and (3) a hard zincblende (cubic) form (CBN) similar to cubic diamond.
The three BN crystal structures may be visualized as formed by the stacking of a series of sheets (layers) of atoms. In the low pressure graphitic structure the stacking layers are made up of planar fused hexagons (like bathroom tile) in which the vertices of the hexagons are occupied alternately by boron and nitrogen atoms and stacked vertically such that the B and N atoms also alternate in the stacking [001] direction as illustrated in FIG. 1A. In the more dense CBN and WBN crystal structures the atoms of the stacking layers are puckered out-of-plane and the two dense structures result from variation in the stacking of the layers. As illustrated in FIGS. 1B and 1C, the layer stacking sequence of the CBN and WBN structures can therefor be symbolized as . . . A B C A . . . and . . . A B A B . . . respectively.
In HBN and WBN crystals the layers are stacked along the [001] direction (i.e. the c crystallographic axis is perpendicular to the layers) whereas in the CBN crystal the layers are stacked along the [111] direction. These layers are referred to as hexgonal stacking layers or planes. In HBN, bonding between the atoms within the layers is predominantly of the strong covalent type, but with only weak van der Waals bonding between layers. In WBN and CBN, strong, predominantly covalent tetrahedral bonds are formed between each atom and its four neighbors.
Hard phase BN compacts are of two general types: a cluster compact and a composite compact.
A cluster compact is defined as a cluster of abrasive crystals bonded together either (1) in a self-bonded relationship, (2) by means of a bonding medium disposed between the crystals or (3) by means of some combination of (1) and (2). Reference can be made to U.S. Pat. Nos. 3,136,615 and 3,233,988 for a detailed disclosure of certain types cluster compacts and methods for making same.
A composite compact is defined as a cluster compact bonded to a substrate material, such as cemented tungsten carbide. The bond to the substrate can be formed either during or subsequent to the formation of the cluster compact. Reference can be made to U.S. Pat. Nos. 3,743,489 and 3,767,371 for a detailed disclosure of certain types of composite compacts and methods for making same.
Known process for making CBN compacts can be generally classified in four categories and are so defined as used herein as follows: (1) catalytic conversion process, a one-step process in which a catalyst metal or alloy aids in the transition of HBN to CBN simultaneously with the formation of the compact; (2) bonding medium process, a two-step process in which the first step comprises the conversion of HBN to CBN and the second step comprises the formation of a compact from cleaned CBN crystals mixed with a metal or alloy which aids in the bonding of the CBN into a compact; (3) direct sintering process, a two-step process which is the same as process (2) except that compact is formed without addition of metal or alloy to aid in bonding CBN crystals; and (4) direct conversion process, a one-step process in which substantially pure HBN is directly transformed to a CBN compact without the aid of a catalyst and/or a bonding medium.
The catalytic and bonding medium processes are generally disadvantageous because the catalysts and bonding medium are lower in hardness than CBN and are retained in the resultant mass which reduces the hardness and abrasive resistance of the masses. Particular reference can be made to U.S. Pat. No. 3,233,988, col. 4, line 3, through col. 6, line 41 and to U.S. Pat. No. 3,918,219 for a more detailed discussion of catalytically formed CBN compacts and to U.S. Pat. Nos. 3,743,489, 3,767,371 for the details of CBN compacts utilizing a bonding medium.
The direct conversion process, while theoretically possible, has been found, in practice, to have high losses because it is difficult to consistently achieve a sufficient number of crystal to crystal bonds distributed uniformly throughout the compact. Without such, the strength and density of the compact are less than ideal.
The direct conversion under static pressure conditions of HBN to the more dense wurtzitic or cubic (zincblende) phases at pressures of 100 kbars and above is described in detail in J. Chem. Phys., 38, pp. 1144-49, 1963, Bundy, et al., and in U.S. Pat. No. 3,212,852. A disadvantage of this method is that in the pressure range above about 100 kbar the effective reaction volume is limited which limits the size of the converted polycrystalline compact products.
More recently, numerous reports and patents have been published concerning the direct conversion of HBN to CBN cluster compacts at pressure below 100 kbar. Representative of these publications are:
1. Wakatsuki et al., Japanese Pat. No. Sho 49-27518. PA1 2. Wakatsuki et al., Japanese Pat. No. Sho 49-30357. PA1 3. Watasuki et al., Japanese Pat. No. Sho 49-22925. PA1 4. Wakatsuki et al., U.S. Pat. No. 3,852,078. PA1 5. Wakatsuki et al., "Synthesis of Polycrystalline Cubic Boron Nitride", Mat. Res. Bull., 7, 999-1004 (1972). PA1 6. Ichinose et al., "Synthesis of Polycristalline Cubic BN (V)", Proceedings of the Fourth International Conference on High Pressure, Kyoto, Japan (1974), pp. 436-440. PA1 7. Wakatsuki et al., "Synthesis of Polycrystalline Cubic Boron Nitride (VI)", Proceedings of the Fourth International Conference on High Pressure, Kyoto, Japan (1974), pp. 441-445. PA1 8. Sirota, N. British Pat. No. 1,317,716, "Process for the Production of Cubic Boron Nitride", May 23, 1973.
Publications Nos. 1 through 7 report direct conversion occurs at pressures&gt;50 kbar (preferably 60 kbar and above) and temperatures&gt;1100.degree. C., while Publication No. 8 reports conversion at pressures of 60 l kbar and higher over the temperature range from 1800.degree. C. to 3000.degree. C.
The publications generally used HBN powder as the starting material. Two publications (Pubs. 6 and 7) reported the use of pyrolytic boron nitride (PBN) as the starting material. Reference can be made to U.S. Pat. Nos. 3,152,006 and 3,578,403, respectively, (which are hereby incorporated herein by reference) for a more detailed description of PBN and R-PBN and acceptable processes for making it.
Publication No. 6 reports the use of PBN as a starting material for the synthesis of CBN cluster compacts in a direct conversion process practiced at a pressure of 69 kbars and a temperature of between 1800.degree. C. and 1900.degree. C. The resultant product (TABLE 1, p. 436) was characterized as a "soft mass" having varying amounts of unconverted HBN.
Publication No. 7 also reports the use of PBN as a starting material for the synthesis of wurtzitic boron nitride (WBN) and CBN. There were no reported results of the successful formation of either WBN or CBN using PBN as a starting material. See TABLE 1, p. 442.
PBN is a low pressure form of HBN made typically by thermal composition of BCl.sub.3 +NH.sub.3 vapors on a graphite substrate. As deposited, it has a high purity of 99.99+%, a density between about 2.0 and 2.18 g/cm.sup.3 (compared to 2.28 for crystalline HBN), a crystalline size between 50 and 100 A and a preferred crystallite orientation between 50.degree. and 100.degree. in the [001] direction (c-axis). The structure of PBN, as with analogous pyrolytic carbon in the carbon system, is not well understood. Various models have been proposed to explain the structure of PBN and pyrolytic carbons. According to one of the more popular models, termed turbostratic state, the B and N atoms form more or less parallel stacks of fused hexagon graphite BN like layers, but with stacking being random in translation parallel to the layers and random in rotation about the normal to the layers. Other models emphasize imperfections and distortion within the layers. The increased interlayer spacing in the pyrolytic materials (3.42 A for PBN compared to 3.33 A for crystalline HBN) is attributed primarily to the disorder in the stacking direction resulting in attenuation of the weak van der Waals attraction between the layers.
Although highly disordered, PBN is not completely devoid of crystallographic order (not amorphous). There is, though imperfect, organization of the B and N atoms into graphite-like layers: it is the ordered stacking arrangement of the layers which is most conspicuously absent. Extensive structural transformation is required to convert pyrolytic BN to the HBN structure shown in FIG. 1.
The "as deposited" type of PBN will be referred to hereinafter as unrecrystallized PBN (U-PBN).
Another known type of PBN is recrystallized PBN (R-PBN). It is formed by compression annealing of PBN and has a theoretical density of 2.28 g/cm.sup.3, a highly crystalline structure with an interlayer spacing of 3.33 A, a purity of 99.99+%, and a preferred crystallite orientation of about 2.degree. in the [001] direction (c-axis).
Each type of PBN is made and commercially available in the form of a solid continuous sheet having the hexagonal stacking planes of each crystallite aligned with major planes of the sheet to the degree of preferential orientation. Thus the hexagonal stacking planes (001) of U-PBN are disposed at angles varying between about 50.degree. and 100.degree. with major planes of the sheet, while the (001) planes of the R-PBN are disposed at angles varying between about 2.degree. or less with the major planes of a sheet.
R-PBN is further described in U.S. Pat. No. 3,578,403 which is hereby incorporated by reference herein.
PBN can be also classified as either "substrate nucleated" or "continuously renucleated". Substrate nucleated PBN is characterized as material substantially free of co-deposited gas-phase formed particles which act as new nucleation sites. Continuously renucleated material is characterized by the presence of co-deposited gas-phase formed particles which result in continuous renucleation during the deposition process. The concentration of co-deposited gas-phase formed particles and thus the degree of renucleation is reflected in the size of the growth cones developed during the deposition process. Large growth cones are characteristic of substrate nucleated material and is thus associated with a low degree of renucleation, and vice versa. The growth cone structure can be observed under low power magnification. The terms "substrate nucleated" and "continuously renucleated" PBN define more or less end point types of microstructure. A range of the microstructures exist between the continuously renucleated microstructure containing a high concentration of co-deposited gas-phase formed particles to the substrate nucleated structure free of co-deposited particles.
Also, the aforementioned U.S. Pat. No. 3,212,852, col. 10, lines 19-24, discloses the use of PBN as a starting material in direct conversion processes practiced at pressures above 100 kbars.
It has been found through experiment that the cluster compacts produced in accordance with the teaching of the foregoing prior art publications still fail to achieve desired performance levels in tests designed to measure the effectiveness of such compacts for cutting tool inserts.
Additionally , the trend to miniaturization in electronics has led to the need for improved heat dissipating substrates for solid state devices. For example, in nearly all microwave devices, heat generated during operation leads to decreased efficiency; and dissipation of the heat generated is the critical factor limiting operation. A commonly used heat sink material oxygen free high thermal conductivity copper has a thermal conductivity of approximately 4 W/cm.degree.C. at room temperature. For applications where excellent dielectric properties are required, sintered beryllium oxide is commonly used even though its thermal conductivity is only about one-half that of copper. A combination of high thermal conductivity and good dielectric properties is highly desirable in a new substrate material.
Type IIa single crystal diamond has the highest room temperature thermal conductivity of any known material and is currently being used to a limited extent for some microwave devices. Known applications for improved heat dissipating diamond substrates range from heat sinks for solid state microwave generators, such as Gunn and IMPATT diodes, through solid state lasers, high power transistors and integrated circuits. It is not extensively used because of cost and difficulty in shaping.
A high thermal conductivity material, less costly than Type IIa single crystal diamond would be highly desirable if it also had good dielectric properties and could be molded in larger pieces than Type IIa diamond.
In addition to diamond, CBN has been suggested as a possible dielectric heat sink material. In Slack, J. Phys, Chem. Solids 34, 321 (1972), pure single crystal CBN was predicted to have a room temperature thermal conductivity of .about.13 W/cm.degree.C. Until recently maximum values of only .about.2W/cm.degree.C. have been reported for sintered CBN compacts. In Japanese Patent No. 50-61413, however, thermal conductivity values as high as 6.3 W/cm.degree.C. for isotope enriched sintered CBN compacts have been reported compared with 1.7 W/cm.degree.C. for sintered compacts of naturally occurring isotope concentration.
Accordingly, an object of this invention is to provide strong, abrasive resistant CBN cluster compacts with improved performance characteristics.
Another object of this invention is to produce large sized CBN cluster compacts by the direct conversion of HBN under high pressure and high temperature (HP/HT) conditions.
Another object of this invention is to produce CBN cluster compacts in sufficiently large sizes to be useful in material removal applications and at HP/HT conditions which are more economical.
Another object of this invention is to prepare CBN cluster compacts having a room temperature (300.degree. K.) thermal conductivity (k) greater than 2 watts/cm.degree.K and preferably greater than 6 watts/cm.degree.K.
Another object of this invention is to prepare CBN cluster compacts of high thermal conductivity by direct conversion of HBN to CBN in which the crystallite size is larger than the room temperature phonon mean free path length and the thermal resistance between grains (crystallites) is not increased by grain boundary oxide contamination.
Another object of this invention is to economically produce CBN cluster compacts having thermal conductivity values suitable for electronic heat sink application.
Another object of this invention is to produce high thermally conductive cluster compacts of high electrical resistance, low relative permittivity and low dielectric loss tangent.
Another object of this invention is to produce high thermal conducting polycrystalline CBN compacts free of secondary binder or sintering aid phases.
Another object is to produce polycrystalline CBN compacts free of impurities (particularly oxygen and nitrogen impurities) which would act as phonon scattering centers and thus limit the thermal conductivity.